System for continuous preparation of terephthalic acid



Feb. 23, 1965 R. H. BALDWIN SYSTEM FOR CONTINUOUS PREPARATION OFTEREPHTHALIC ACID Original Filed April 22, 1959 IN VEN TOR Richard h.Baldwin United StateS Patent Office 3l70,768 Patented Feb. 23, 1965 3Claims. (Cl. 23-263) This invention relates to the cornmercialpreparation of terephthalic acid, and more particularly is concernedwith the provision of an integrated process for the oxidation ofparaxylene to prepare high purity terephthalic acid.

The primary object of the present invention is to provide a system foroxidizing paraxylene to terephthalic acid using molecular oxygen as theoxidizing agent and conducting a reaction in the liquid phase; in theinventive system means are provided for initiallypurifying andrecovering the terephthalic acid product using simplified purificationtechniques. An additional object is to provide, in the above system,integrated facilities for recovering and recycling the lowermonocarboxylic acid used as a liquid oxidation reaction mediurn and,optionally, to recover and recycle the Oxidation catalyst. A furtherobject is to provide an integrated systern for conducting the oxidationand product recovery operations in a manner which obviates the elaborateand expensive equipment hitherto required for prior art processes. Otherand more particular objects will become apparent as the detaileddescription proceeds.

In practicing the present invention, a substantially pure paraxylene,i.e., a paraxylene of at least about 95% and preferably about 98% orhigher purity is xidized with air in the liquid phase. The reaction iscomducted in a zone"maintained at a temperature in the range of about325 to 475 F. and preferably 350 to 450 F. and a pressufe elfek:tive tomaintain a liquid phase in the reaction Zone, the desirable pressurebeing in the range of 50 to 500 p.s.i. and preferably 200 to 350 p.s.i.,for an average contacting period of about .2 to 2 hours o1 more. Alsopresent in the reacton zone as a liquid reaction medium is a saturatedmonocarboxylic acid having from 2 to 8 carbon atoms in the molecule,illustratively acetic acid, which functions as solvent for catalyst andfeedstock. After completion of the oxidation reaction, crude crystals ofterephthalic acid are physically separated from the reaction mixture;the crude crystals are purified by washing them with dehydratedsaturated monocarboxylic acid, obtained as hereinafter described. Wherehigher purity terephthalic acid is required the Crude crystals may bereslurried with at least a major portion of the dehydratedmonocarboxylic acid, from which slurry the terephthalic acid crystalsrnay be physically separateid by suitable rneans such as centrifugation.Desirably, these latter crystals may be re-washed With a minor ortion Cfdehydrated monocarboxylic acid. The wash liquor from this operation isemployed to pre-wash the crude terephthalic acid crystals initiallyseparated from the reaction mixture before reslurrying or washing thecrystals with the major portion of dehydrated monocarboxylic acid. Thedehydrated monocarboxylic acid employed as wash liquor according to anyof the foregoing Variations is obtained by distillation of the liquidportion of the reaction mixture, by rneans cf which water and adistillation bottorns or residuefraction =are exclude'd. This bottomsfraction contains Oxidation catalyst as weil as oxidation intermediatesand by-products; the invention also contemplates recovering oxidationcata- 1 yst from the bottoms f raction by selectively extracting acidicintermediates and byproductsfrom the bottoms fraction With an aqueoussolution of a monovalent alkali hydroxide, for example caustic,arnmonia, potassium hydroxide, etc. The catalyst, which rernains as aninsoluble material and which is free from the intermediates andbyproducts, may be recycled to the oxidation reaction zone.

The invention will be more clearly understood from the followingdetailed description of a specific sommercial exarnple read inconjunction with the accompanying drawing which is a schematic flowsheetof the improved 'process. The process is designed toproduceCommerc'ially pure terephthalic acid. Process utilities and auxiliariessuch as umps, valves, gages, etc. have been omitted from the drawing andspecification for simplicity, and those skilled in the art will readilyperceive the nature and location cf these items in the plant.

Oxidation is effected either batchwise or continuously in reactor 1,Which is constructed of corrosion resistant metals and is provided withrotary mixer 19. The paraxylene feed, preheated to 300350 F. inpreheater 3, enters the system through line 2 at a rate of 1000 poundsper hour. The saturated monocarboxylic acid is employed in a proportion01 from about 1:1 to about 5:1 Weight per weight of paraxylene; in thisexample it is introduced at a rate of 2000 pounds per hour expressed asacid, the balance being water, and enters through line 4 via an optionalpreheater, not shown. Catalyst, in

the form of a slurry or solution in acetic acid, is. stored in tank 5and fed into reactor 1 through line 8. The oxidizagent, air in thiscase, is introduced thrugh line 6 by cornpressor 7 at a rate sufiicientto complete the oxidation reacti=on in about one hour.

' The present invention is an improvenient 011 the fundamental heavymetal-bromine catalyst system recently developed by others, and nonovelty is claimed per se in the catalyst er in the catalyst pi*omoter.C[-Iowever the catalyst system is an essential feature of the presentinvention as it largely eliminates the substantial quantities of tarsand incornpletely-oxidiied paraxylene characteristic of prior-artcatalyst systems, and hence greatly reduces the equipment requirementsin the plant. The catalyst is one or more of the heavyjrnetals, whichinclude the lanthanide and actinide rare ei1rlths, preferably in a formwhich is soluble in the reaction mixture. The catalyst metal mayadvantageously be selected from the group manganese, cobalt, nickel,chromium, vanadium, molybdenum, tungsten, tin and cerium, and the metalmay be employed as an element, in ionic forrn, er as a constituent of anorganic compound; it is preferably in the form of a metal acetate oracetate hydrate. The promoter is a substance capable of afiordingbromine, the promoter being elemental bromine or an organic or inorganicbromine-con taining cornpound, Thus the bromine may be added in theform, illustratively, of potassium brornate, arnrnonium bromide, benzylbromide, tetrabromoethane, mangnese bromide, 01' the like. Theproportions of catalyst and promoter rnay be prkeferably stoichiometricas exemplified by manganese bromide, MnBr and generally are in the rangeof 1:10 to 10:1 atoms of catalyst metal per atom of brornine. The amountof catalyst employed is usually in the range of about 0.01 to 10% ormore 0f paraxylene, and is preferably in the range Cf about 0.2 to 2% byweight, all figures being expressed on the basis of elemental metal t0elemental bromine. In certain instances a mixture of two or morecatalysts is desirably employed, the preferred mixe d metal catalystsbeing mixtures of manganese in the formof its biomide, hydroxide, or acetate With cobalt broinide,v cobalt versene, Cobalt acetate, or thelike. In the system described herein a mixedmanganese and cobalthydroxide catalyst is suspen'ded in acetic acid in tank 5 and introducedto reactor 1 through line 3, with makeup catalyst being added asrequired. The bromine-afiording substance is tetrabromoethane.

During the oxidation, the re-action between paraxylene and molecularoxygen Which occurs in reactor 1 evolves considerable heat of reactionwhich manifests itself in boiling 011 acetic acid, water and someparaxylene. These vapors together with non-condensible gases passthrough line 9 and enter reactor overhead condenser 10 and via line 11into entrainment separater 12. There the coudensed acetic acid, waterand paraxylene separate as one or more liquid phases which are returnedto reactor 1 through leg 13 where they serve to control the temperaturein reactor 1. Since however condenser 10 is generally not eifective tocondense all of the valuable acetic acid and paraxylene vapors, vaporline 14 olf entrainment separator 12 asses through reactor vent gasscrubber 15 which may be a tower packed with Berl saddles where the gasis scrubbed With 200 pounds per hour of water introduced into scrubber15 through line 16. The vent gas leaves at line 18, and the rich liquorcomprising dilute aqueous acetic acid is withdrawn through line 17 andconducted to solvent dehydration column 16, to be described presently.

The reaction mixture at the conclusion cf the Oxidation is obtained fromreactor 1 through line 20 and sent to eflluent surge tank 21. Thismixture is in the form of a tl1ick slurry of terephthalic acid crystals,and is kept in suspension by recirculating via recycle line 22, pump 23,and lines 24 and 25. Vapors from etlluent surge tank 21 pass throughline 27, through overhead condenser 28 which condenses and returnsacetic acid and water vapors back to tank 21, and through line 29 to lowpressure vent gas absorber 59 for acetic acid removal by werterscrubbing. In absorber 59, 100 pounds per hour of water introducedthrough line 60 scrubs acetic acid vapors from the gas, and the outletgas is vented through line 61 while the rich liquor passes via line 62to solvent dehydration column 66.

after is withdrawn from solids reslurrying drum and sent to centrifuge39 via line 38. In centrifuge 39, wet terephthalic acid crystals areseparated frorn magna, and the mother liquor is sent via line 43 andsurge tank 44, back to the reactor via line 4. This mother liquorcomtains most 0f the impurities originally present in the crudeterephtnalic acid. While still in centrifuge 39, the terephthalic acidcrystals are washed with a freish stream of 95% acetic acid comprising aminor amount of the acetic acid dehydrated in solvent dehydration column66 and stored in surge drum 55; this wash acid is then separated fromthe crystals in centrifuge 39 and used as wash in first stage centrifuge30 where it provides an initial purification for the crude terephthalicacid originally separated from the bot reaction mixture.

The washed crystals then leave centrifuge 39 through line 45 and passinto vibration drier 46 where inert gas frorn flue gas generator 49 issupplied through line 47 contacts the crystals in a countercurrentdirection and evaporates the adhering acetic acid. Inert gas and aceticacid vapors leave Vibration drier through line 48 where they join asimilar stream frorn first stage centrifuge 30 and frorn second stagecentrifuge 39 (through line 51) for acetic acid recovery.

The dry terephthalic acid which leaves vibration drier 46 as a crystalpowder stream is rernarkably pure and is virtually free from anyimpurities. lt carx generally be used in applications requiring highdegrees of terephthalic acid purity; where exceptional purity standardsare imposed, additional treatment may be employed. The terephthalic acidfrom the present system is obtained at a rate of 1300 pounds per hour,which is a yield cf 7 130 weight percent of paraxylene fed to thesystem.

From surge tank 21, the rnain reactor efiluent stream is pumped whilebot through line 26 to first stage centrifuge 30 which separates crudewet terephthalic acid crystals from the reaction mixture. Because of the10W solubility of terephthalic acid in the reaction mixture,centrifugation of the bot liquor is effective in recoveringsubstantially all of the acid while retaining such impurities ascatalyst and isophthalic and o-phthalic acid in the centrifugate. Themother liquor is sent through line 34 to the solvent flash distillationcolumn 63 to begin acetic acid and catalyst recovery. The centrifugedcrystals are washed While in centrifuge 30 With from about 0.1 to 1.0pound of about 95% acetic acid per pound of crystals, which acetic acidis withdrawn as the efiluent wash liquor from second stage centrifuge39, and this stream is ultimately conducted through 1ine 34 to solventflash distillation column 63. It Will be observed that this first stagewash serves to remove gross amounts of adhering liquid reaction mixturefrom the crude terephthalic acid crystals. The washed crystals arepassed through line 31 to solids reslurrying drum 35 which is equippedwith an agitator as shown and with vertical plate bafiles; Here fromabout 1.0 to about 5.0 pounds of about 95% acetic acid per pound ofcrystals, which is obtained from solvent dehydration column 66 viarecycle solvent surge drum 55 and lines 56 and 37, is adcled. Meanwhile,a stream of inert gas, i.e., fiue gas produced in inert gas generator 49by burning natural gas supplied through line 50, is fed into solidsreslurrying drum 35 through line 36, and passes through line 31, throughfirst stage centrifuge 30, and ultimately through line 32, knock outcondenser 53, line 58, and low pressure vent gas adsorber 59, tomaintain a slight positive pressure on the systern.

The slurry of terephthalic acid in acetic acid is rnaintained in drum 35for several minutes at about 150-300" F. e.g., 200 F., with continuousagitation and there- Returning to first stage centrifuge 30, the aceticacid wash liquor and reactor eflluent liquid cenlrifuged fron1 theterephthalic acid crystals is sent through line 34 t0 solvent flashdistillation column 63, operating at a pressure of between aboutatmospheric and about pounds per square inch gage pressure, e.g. 25p.s.i.g. and at a reboiler temperature of 350 F. for 25 p.s.i.g.operation. In column 63, a strearn containing substantially all of thewater and acetic acid is distilled overhead, and after being condensedin overhead condenser 64 is split into a reflux stream and an overheadstream comprising dilute aqueous acetic acid at a rate of 1982 lbs./hourof acetic acid and 402 pounds per hour of Wetter, the acetic acid beingherein expressed as acetic acid and 5% water. A simplified embodiment ofsolvent fiash distillation colurnn 63 includes the elimination ofcondenser 64 and the direct transfer of vapors frorn column 63 tosolvent dehydration column 66; this eliminates one condenser andreceiver and reduces the 1oad on the reboiler cf column 66. The bottomsfrom solvent flash distillation column 63 cornprises heavy n1ctaloxidation catalyst, bromine prornoter, oxidation intermediates andby*products, and a traoe of terephthalic acid.

The overhead distillate from colurnn 63 cornprising water and aceticacid is pased through line 65 to solvent dehydration column 66,operating at about 8-30 pounds per square inch absolute pressuredesirably about 1620 p.s.i.a. Here watcr is distilled overhead while thedehydrated 95% acetio acid is obtained through linc 67 as a bottornsproduct. The overhead distillate from column 66 is condensed incondenser 68 and thenet reflux sent through line 69 to slop facilities.Solvent dehydration colurnn 66 may optionally contain any one cf theknown acetic acid-werter azeotroping agents such as methyl isobutylketone for improved fractionation efiiciency.

Returning momentarily to solvent flash distillation column 63, thenormally-solid distillation bottoms from this column comprisingcatalyst, bromine prornoter, acidic reaction by-products andintermediates, and any unrecovered terephthalic acid is withdrawnthrough line 66 and sent to kettle 88. Kettle 88 is provided witlr ascrapper-type agitator to prevent the accmulation of through line 86 forthe purpose of dissolving intermediates, by-products and terephthalicacid, leaving the heavy metaloxidationcatalyst in the form ofaninsoluble residue, presumably either :hydroxide er ter'ephthlate.salts of the catalyst. Kettle88 is maintained under vacuum with thevapors passing through line 7lvacuumpump exhaust condenser 72, and lowpressure vent gas absorber 59 via a line, not shown. Liquid condensed bycondenser 72 is sent via line 74 back to solvent flash distillationcolumn 63.

The oxidation catalyst suspended in caustic is trarisferred from kettle88 via line 75 fo plate-and-frarne filter press 76 where the catalyst isseparated and recovered as a re-usable filter cake. 'This cake isreturned to preparatior1 tank 5 where it is dispersed in acetic acidforultimate return to oxidation reactor 1. It is rr'1anifest that heavymetal oxidation catalysts employed in the oxidation of any aromaticcompou nd to produce aromatic carboxylic acids may be separted fromtheir admixture With acidic reaction by-prbducts bymeans of extractionwith aqueous solutions cf monovalent alkali hydroxides.

The mother liquor from filter press 76 is transferred t0 by-product acidspringing tank 7 9, Where 40 Bzium nitric acid is introduced to convertacidic reaction by-products to the water-insoluble or ganic acid forms.Theseorganic acids, comprising terephfchalic acid plus intermcdiates andby-products are recovered from by-product filtei press 32 via line83Qwhile the mothejr liquor passes through line 84 From the foregoingdescription it is manifest that the present invention provides anexceedingly valuable commercial system for producing high purityterephthalic acid frorn substantially pure paraxylene. The combinationQf. high-efliciency catalytic airoxidation, terephthalic acidpurific'ation, and iritegrated acetic acidrecovery and recycle, rendersthe here tofore difiicult preparation of high purity terephthalic acidbOth convenient and economical. Furthermore the recovery and recycle cfhe avy metal tent on the particular process conditionsutilizedthrougghout the plant. Hence the conditions are to be'considered ope rable for the specific example desc;ibed, although notthrough oxidizing a feedstock consisting essentially of necesarilyoptimum for all operations andfor other monoczirboxylic acids such zisprofpionic, etc. 4

This appli iation is a division of my co;iending application S.N.808,150, filed'April 22 1959, n0w abandoned.

Hving described the preferred formof the present invention, what isclairned is:

'1. Apparatus for making "purified terephthalic acid paraxylene Withm0lccular oxygen in the presence of dehydrated saturated monocarboxylicacid having from two to eight carbon atoms, inclusive, in the molecule,heavy metal oxidation catalyst, and a bromine-aifording substance all ofwhich comprise mixturefor reaction; which comprises: an oxidati-onvessel fabricated from corrosion resistant metal having .means forreceiving mixture for reaction, removing liquid reaction mixturecontaining terephthalic acid solids, and means for removing vapors andgases therefrom; a condenser; transfer means for comducting said vaporsand gases to said condenser; transfer means for returning condensatet0'Slid oxidation vessel; solid-liquid phase separation rneans forseparating crucle terephthalic acid crystals frorn the liquid pqrtion ofthe reaction mixture; distillatioflmeans for separating said liquidportion into a water overhead fraction, a dehydrated saturatedmonocarboxylic acid fraction, and a.

distillation bottoms fraction containing catalyst and acidie reactionbyproducts andintcrmediatias; meai1s for washtwo centrifuges, withconnectiohs for supplying to the first centrifuge a portion of thesecond centrifuge wash the oxidation vessel and for conducting the otherportion With dehydrated saturated mo'nocarboxylic acid in two portions,and connections for conducting one portion to t0 the first c6ntrifuge.

3. APParatus of claim 2 including a reslurrying vessel between said twocentrifugeswherein the initially washed terephthalic acid crystals areslurried with the other por tion of dehydrated saturatedmonocarboxylicacid.

References Cited in the file of this patent UNITED STATES PATENTS2s45144 Green et a1. Mar. 13, 1951 Spille r et al Nov. 29, 1960

1. APPARATUS FOR MAKING PURIFIED TEREPHTHALIC ACID THROUGH OXIDIZING AFEEDSTOCK CONSISTING ESSENTIALLY OF PARAXYLENE WITH MOLECULAR OXYGEN INTHE PRESENCE OF DEHYDRATED SATURATED MONOCHARBOYLIX ACID HAVING FROM TWOTO EIGHT CARBON ATOMS, INCLUSIVE, IN THE MOLECULE, HEAVY METAL OXIDATIONCATALYST, AND A BROMINE-AFFORDING SUBSTANCE ALL OF WHICH COMPRISEMIXTURE FOR REACTION; WHICH COMPRISES: AN OXIDATION VESSEL FABRICATEDFROM CORROSION RESISTANT METAL HAVING MEANS FOR RECEIVING MIXTURE FORREACTION, REMOVING LIUQID REACTION MIXTURE CONTAINING TEREPHTHALIC ACIDSOLIDS, AND MEANS FOR REMOVING VAPORS AND GASES THEREFROM; A CONDENSER;TRANFER MEANS FOR CONDUCTING SAID VAPORS AND GASES TO SAID CONDENSER;TRANSFER MEANS FOR RETURNING CONDENSATE TO SAID OXIDATION VESSEL;SOLID-LIQUID PHASE SEPARATION MEANS FOR SEPARATING CRUDE TEREPHTHALICACID CRYSTALS FROM THE LIQUID PORTION OF THE REACTION MIXTURE;DISTILLATION MEANS FOR SEPARATING SAID LIQUID PORTION INTO A WATEROVERHEAD FRACTON, A DEHYDRATED SATURATED MONOCARBOXYLIC ACID FRACTION,AND A DISTILLATION BOTTOMS FRACTION CONTAINING CATALYST AND ACIDICREACTION BYPRODUCTS AND INTERMEDIATES; MEANS FOR WASHING THE CRUDETEREPHTHALIC ACID CRYSTALS WITH DEHYDRATED SATURATED MONOCARBOXYLICACID, CONNECTIONS FOR SUPPLYING SAID DEHYDRATED ACID TO SAID MEANS FORWASHING THE CRUDE TEREPHTHALIC ACID CRYSTALS, AND CONNECTIONS FORCYCLING THE DEHYDRATED ACID AFTER WASHING TO THE OXIDATION VESSEL; ANDMEANS FOR DRYING THE WASHED TEREPHTHALIC ACID CRYSTALS TO RECOVER APURIFIED AND DRIED TEREPHTHALIC ACID PRODUCT.